check return value of unlink(2) when trying to remove an existing HISTFILE,

since mksh(1) did go into an infinite loop if that fails first

bug spotted, initial patch and help drafting a test case
From: Decklin Foster <decklin@red-bean.com>

note there are more instances of unlink(2) and others (like chmod(2), as
spotted by flawfinder) which aren’t checked… but at least the other case
of unlink(2) use in histrap.c doesn’t cause any trouble (I think)

histrap.c

@@ -3,7 +3,7 @@
 
 #include "sh.h"
 
-__RCSID("$MirOS: src/bin/mksh/histrap.c,v 1.64 2008/06/08 17:16:25 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/histrap.c,v 1.65 2008/07/06 22:41:08 tg Exp $");
 
 /*-
  * MirOS: This is the default mapping type, and need not be specified.
@@ -677,17 +677,27 @@ hist_init(Source *s)
 			if (base != (unsigned char *)MAP_FAILED)
 				munmap((caddr_t)base, hsize);
 			hist_finish();
-			unlink(hname);
+			if (unlink(hname) /* fails */)
+				goto hiniterr;
 			goto retry;
 		}
 		if (hsize > 2) {
+			int rv = 0;
+
 			lines = hist_count_lines(base+2, hsize-2);
 			if (lines > histsize) {
 				/* we need to make the file smaller */
 				if (hist_shrink(base, hsize))
-					unlink(hname);
+					rv = unlink(hname);
 				munmap((caddr_t)base, hsize);
 				hist_finish();
+				if (rv) {
+ hiniterr:
+					bi_errorf("cannot unlink HISTFILE %s"
+					    " - %s", hname, strerror(errno));
+					hsize = 0;
+					return;
+				}
 				goto retry;
 			}
 		}